105.2 Customize or write simple scripts

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Description

Candidates should be able to customize existing scripts, or write simple new Bash scripts.

Key Knowledge Areas:

• Use standard sh syntax (loops, tests).
• Use command substitution.
• Test return values for success or failure or other information provided by a command.
• Execute chained commands.
• Perform conditional mailing to the superuser.
• Correctly select the script interpreter through the shebang (#!) line.
• Manage the location, ownership, execution and suid-rights of scripts.

Terms and Utilities

• for
• while
• test
• if
• read
• seq
• exec
• ||
• &&

combining commands

If you want to run more than one command in oneline, separate them by a ;. So the cd /tmp; ls will run the cd /tmp and then ls. But there are more advanced usages too.

You can use && (as logical And) and || (as logical OR) chaining. In case of And, the execution will stop as soon as the first one fails to execute. In case of Or, the next command will only run if the first one fails. Just like in logic gates.

Confusing? Let me explain again. The system will always try to evaluate the outcome of your chain. So if you have A && B then A fails, the system does not need to test B (because the overall result will be False anyway). Same logic works for A || B || C. If A works fine, the overall evaluation will be True so no need for testing B or C. But if A fails, system will try B and if B fails, we have to try C.

We use chaining to create logical flows. For example:

$ cp backup.gzip /backups/ && rm backup.gzip

works like this: copy the backup to this location and delete the file ONLY IF the previous copy was successful.

Shell Scripts

We can combine shell commands or programs and or some logic or loops to write large or small scripts. Shell scripts are useful when you want to automate some tasks in one larger command. Say a shell script called "do_backup.sh" may compress some files into a file, rename this file based on the current date and time and then save it on a remote site and at last delete the ones older than 1 month.

Shebang

There is a line at the beginning of scripts which starts with #! and continues with an executable who needs to run this script. It is called shebang and as mentioned, tells the shell which interpreter must be used to run this script.

Note: In many programming languages (including Bash & Python), a # at the beginning of a line in script indicates comments. Do not confuse it with the Shebang (#!)

Commonly we run shell scripts using #!/bin/bash or #!/bin/sh

The rest of a shell script can use most of the commands you already know in addition with some more "programming" specific commands like loops, tests and such. Here is a primitive sample:

#!/bin/bash

echo
echo "We are learning! Wowww..."
echo

The sh is a more basic shell but it is compatible with most of the things we talk about. There are also other options like zsh and csh but the LPIC is based on bash.

Variables

Already seen in the previous section. You can define variables like this VARNAME=VALUE. Here is a sample:

#!/bin/bash

NAME=Jadi

echo
echo "$NAME is learning! Wowww..."
echo

Note: you can also do NAME="Jadi the geeking guy" if you need to have spaces in your values

If you want to access the command line arguments in your shell script, use $1, $2, .... You can find the number of command line arguments via $# variable.

Command substitution

Sometimes you need to save the output of a command in a variable or use it in some way. To do so you can use $(command) or simply `command` (Backtick). Look at these two samples:

➜  FILES=$(ls -1) # the $FILES variable contains the list of all files

➜  ~ date +'%Y%m%d-%H%m'
20230408-1604
➜  ~ touch backup_`date +'%Y%m%d-%H%m'`.tar
➜  ~ ls
backup_20230408-1604.tar

executing scripts

In the Unix/Linux world, file should be executable to be executable! :D To do so use the chmod command with +x. After this step you can run your script by giving its path to shell. Please note that Linux by default does not looks into the current directory so if you are going to run a script at the current directory, you have to run ./script_name.sh.

Another way to run a script is running sh or bash with the shell scripts name as its argument. That is sh my_script.sh.

In both above methods, the scripts runs inside a child bash process and returns back afterwards to the same shell. If you want to override this and replace your current shell with the program you are going to run, use the exec build-in command. The -c switch will run the command in a clean environment.

Conditions

Up to now, we were just running commands one by one. That is not very programmatic. If we are going to have some logic in our programs, we need conditions and loops. First we will cover conditions, using the if command. Its usage is like this:

if [condition]
then
   do something
   do another thing
else
   do new things
   even funnier things
fi

Note: else part is optional, if, then, fi is enough.

Conditions can be TRUE or FALSE. A very simple conditions is if [ "Linux" = "Linux" ]. Silly? I know but wait; we are learning the syntax only. Please give special attention to the spaces and = for checking if two strings are equal.

#!/bin/bash

kernel=$(uname -s)
if [ $kernel = "Linux" ]
then
    echo YES. You are using a Linux
else
    echo "Not a linux :("
fi

The command to check conditions is the test command but since it is used a lot, we have a shortcut for it. Instead of test condition you can write [ condition ].

read

Using read we can read the user input. Look at this:

#!/bin/sh

echo "what is your name?"
read NAME

echo "Hello $NAME"

if [ $NAME = "Jadi" ]
then
    echo "Oh I know you!"
else
    echo "I wish I knew you"
fi
echo "Bye"

You can timeout the waiting using the -t and show a prompt using -p.

if read -t 10 -p "Server address?" SERVER
then
    echo "Connecting to the $SERVER ..."
else
    echo
    echo "Too late!"
fi

loops

In programming loops are used to repeat part of the programs. We have 2 different loops in Bash; for and while. The for loops lets us run part of a program for a specific number of iterations. The while loop is used when we want to repeat part of a program while a specific condition became true.

for

The syntax is like this:

for VAR in SOME_LIST;
do
  some stuff with $VAR
  some other stuff
done

Note: the in, ;, do and done.

On each iteration, the VAR will be equal to one of the SOME_LIST elements. SOME_LIST can be numbers, name of files, words, ...

for NUM in 1 2 3 4 5 6;
do
    echo $NUM
done

But what if you needed to run from 1 to 42? We have the seq command for that. The seq 1 42 or its shorthand {1..42} will let us run a loop 42 times.

We can also use non-numeric variables here. This is a common use case:

for FILE in $(ls);
do
    echo $FILE
    wc -l $FILE
done

while

This is the syntax:

while [condition]
do
    do something
    do anohter thing
done

If your condition stays true all the time, the while loop will run forever. This is called an infinite loop and should be stopped by a Ctrl+C.

This is sample:

VAR=52

while [ $VAR -gt 42 ]
do
    echo VAR is $VAR and it is still greater than 42
    let VAR=VAR-1
done

Note the let usage! If you just say VAR=1 and then VAR=$VAR+1, then VAR will be equal to 1+1 as an string!.

mailing the root user

For sending mail, you need to install mailutils. Then the mail command will send emails. You can send the mail to the root user by issuing this command:

jadi@funlife:~$ mail root
Cc:
Subject: Hi there root
hello there. This is my mail

And root will get this email. She can read it using mail command.

If you need to send emails in a script, just do:

$ echo "Body!" | mail -s "Subject" root

returned values

In the Unix world, the programs return values when they are finished. If you have programmed C, this is the return 0 at the end. Commonly a 0 means successful execution. This return value can be read / examined using $? variable.

jadi@ubuntuserver:/etc/skel$ touch /chert
touch: cannot touch '/chert': Permission denied
$ echo $?
1
$ touch /tmp/11
$ echo $?
0
$ test -e /
$ echo $?
0
$ test -e /nonexist
$ echo $?
1
$ dummycommand
dummycommand: command not found
$ echo $?
127

To return values from your shell scripts, use the exit command; say exit 0 for a successful exit.

Bonus

Just found my old general_backup.sh script and uploaded it to github for you. Nothing fancy but shows you how a general bash script can help you on your daily tasks.